For environment protection purposes, it is desirable to reduce a nitrogen oxide (NOx) exhausted from a gas turbine. As a method of reducing the exhaust amount of NOx, there is a method in which fuel and compressed air are fully mixed (perfectly pre-mixed), and the resulting air-fuel mixture is injected from a fuel injector and combusted. In accordance with this method, since combustion is performed quickly, an increase in a combustion temperature can be suppressed. Therefore, generation of NOx (thermal NOx) due to the increase in the combustion temperature can be suppressed (see Patent Literature 1).